1. Field of the Invention
The present invention concerns the transmission of digital data, especially on an optical link.
2. Description of the Prior Art
In systems for transmitting digital data, especially on an optical link, transmission quality (as measured by the bit error rate at the receiving end, namely the ratio of the number of erroneous bits downstream of the decision operation carried out at the receiving end of any such system to enable said data to be reconstituted to the total number of bits received) depends largely on the performance of the transmitter and the receiver, and in particular on the performance of the optical transmitter and receiver in the case of an optical link.
The characteristic parameters of the transmitter and the receiver (in the case of an optical link, for example, the wavelength and the optical power transmitted, or the avalanche factor of a receiving photodiode) are currently determined, together with other parameters characteristic of the system (including the type of fiber used in the case of an optical link), when the system is designed on the basis of the requirements of the system user (including the length of the link, the bit rate to be transmitted and the maximum bit error rate acceptable at the receiving end) in order to guarantee at the receiving end a bit error rate not more than the maximum acceptable bit error rate specified by the user.
A drawback of this way of setting characteristic parameters of the transmitter and the receiver is that it does not necessarily lead, in use, to an optimum point of operation of the transmitter and/or the receiver, among other things because it does not allow for changes in these parameters with time.
There are transmission systems, including optical transmission systems, in which the send optical wavelength is slaved in use to a reference wavelength determined when the system is designed, as mentioned above, in this instance to locate it within a particular optical window in turn depending on the type of fiber used and the possible use on the link of components such as optical amplifiers.
A drawback of any such control mechanism is that it is relatively costly and complex to implement as, among other things, it requires the use of a highly stable optical source whose wavelength is precisely known. Also, it does not necessarily lead to an optimum point of operation for the optical transmitter in the sense that, being a localized function, it does not allow for the overall effect of the transmission system, i.e. in this instance the conjugate effects on system performance of wavelength drift and possible changes of other transmission system characteristic parameters.
An object of the present invention is a system for transmitting digital data, especially on an optical link, with which the above drawbacks can be avoided.